In general, it is known that two parts are joined together after being formed separately to obtain a closed structure part in the fields of automobiles and consumer electrical appliances. Hydroforming and roll forming are known methods for manufacturing curvilineal closed structure parts with flanges of minimized areas.
All of the peripheral portions of metal plates must be welded before the part is being filled with working fluid in existing hydroforming methods, but Japanese Unexamined Patent Application Publication Nos. 2008-119723, 2000-263169 and 2003-311329 disclose the following methods.
JP '723 discloses a hydroforming apparatus, a hydroforming method, and a product manufactured by the apparatus and method, which provide high productivity by ensuring good sealing in bulge forming of two or more metal plates laid to overlap each other without all of their peripheral portions being welded, and by deep-drawing multiple parts at the same time.
JP '169 discloses a method for manufacturing a closed section bent elongated member, including a multistage roll forming process in which strips are formed into a nearly closed structure, a crimping forming process in which the butting sections of the strips are joined together to make a closed section elongated member, and a bending process in which the elongated member is bent in the moving direction of the strips.
JP '329 discloses a technique for obtaining a product having a twisted portion in closed section out of one piece of material by press forming, and for manufacturing a high-quality lightweight and highly rigid pressed product having a twisted cross section at a low cost.
JP '723 discloses a hydroforming technique in which metal plates can be formed without the occurrence of leaking of working fluid by arranging sliding seals on plate holding surfaces to be brought into contact with one or both of the metal plates and non-sliding seals on the plate holding surface to be brought into contact with the other of the metal plates, whereas in existing hydroforming methods all of the peripheral portions of the metal plates must be welded before the part is filled with working fluid. A welding process before hydroforming can be omitted and the time taken to extract the working fluid can be reduced by using this method. The fact remains, however, that this method still uses hydroforming and the equipment cost is very high. In addition, welding is necessary to complete assembly of a product, which brings about the problem of poor surface accuracy.
JP '169 discloses a roll forming technique in which a closed section bent elongated member is formed by roll forming and crimping joints are formed on the surfaces in the longitudinal direction to omit welding of the butting sections. The cost of this technique may be lower than existing roll forming techniques due to continuous welding for forming a closed section being omitted, but it is difficult to reduce the cost of equipment, because many dies for roll forming are indispensable.
JP '329 discloses a press forming method. First, flanges are formed on the both sides of a blank which is preformed in this method. Then, the preformed part is pressed into a die which is narrower and closer to the final shape than the preformed part. Finally, the preformed part is pressed into a die which has a twisted portion in the longitudinal direction. In this case, a pressed part with a twisted portion having straight vertical walls can be formed, but, if a pressed part has curved vertical walls, a large rolling deformation is generated in the flanges formed by the preforming, which brings about a problem that it is difficult to match the butting sections to each other.
As mentioned above, hydroforming and roll forming are known methods for manufacturing closed structure parts, but those methods are disadvantageous compared to press forming in terms of costs of equipment and production, and press forming can be used only for limited cross sectional shapes.
It could therefore be helpful to provide a method for manufacturing polygonal shape closed structure parts with a bent shape in the longitudinal direction by only using press forming to reduce production cost.